One of the problems with optical switching technology is the intermediate storage of signals. To switch signals that are combined in the time multiplex, the time positions almost always require changing, and the signals must therefore be intermediately stored. With packet switching facilities, which are presently of particular interest in view of future ATM systems (ATM=Asynchronous Transfer Mode), intermediate storage must always be used when two packets, called cells in the ATM, are simultaneously ready for routing to a predetermined channel. Because of the traffic volume statistics, it cannot be excluded that two packets must be simultaneously routed in the same path.
From J. S. Turner's "Design of a Broadcast Packet Network", published in "Proceedings of INFOCOM '86" in April 1986, pages 667 to 675, it is known to combine several lines into a group within an ATM coupling network. Each of the lines of a group run parallel and are fully equivalent to each other. Aside from the fact that the capacity is increased in this instance, it is significant that the statistic traffic volume fluctuations are equalized, which lowers the number of necessary intermediate storage.
An ATM switching stage is known from DE 37 42 941 C2 (corresponds to U.S. Pat. No. 4,922,487, in which a fixed number of cells or packets to be switched are combined in a frame, all packets are divided into equally long packet parts and distributed into subframes. The switching takes place on the basis of subframes in the synchronous time multiplex. This makes use of the fact that all time channels in a line represent equivalent receiver channels, which equalizes most statistical fluctuations. The memory storage are those that must be used anyway with synchronous time multiplex switching. The storage space requirement in the coupling field is reduced by the frame: subframe ratio.
From D. W. Smith's et al "Multidimensional Optical Switching Networks", IEEE 1989, it is known to combine the known multiplex methods, in which wavelength and frequency multiplex belong to the time and space multiplex of the optical switching technology, in order to increase the capacity of an optical switching circuit. The addition of the wavelength or frequency multiplex as another dimension not only increases the capacity of the facility, as is desired in this instance, but it also increases the number of equivalent receiver channels to the same degree. However, very narrow limits have been established for the total capacity of this switching circuit.
A sufficient number of equivalent receiver channels equalizes the occurring statistical fluctuations, so that the intermediate storage can be omitted, except for those required to change the time positions in the time multiplex.
Changing the time position of an optical signal is associated with high costs. A basic possibility of realization consists in converting the optical signal into an electrical signal, to intermediately store it, and to reconvert it into an optical signal at the desired point in time. At least for the present, bistable optical elements are excluded as the intermediate storage of whole ATM cells or whole frames in a synchronous multiplex. The only present possibility to intermediately store optical signals directly in the optical area lies in the use of time links, which are realized by optical fibers. Taking the length of intermediately stored signals into consideration, and the time during which the signals must be intermediately stored, fiber optic lengths on the order of 30 m to 25 km are required.
Furthermore, a conversion in time of an optical signal can basically not be avoided. On the one hand, every synchronization, which is not considered in this invention, is associated with conversion in time. On the other hand, with synchronous multiplex technology, the conversion to a very defined time position is the actual goal of the switching process, while with ATM, the avoidance of the collision in time of two ATM cells is one of the foremost goals of the switching process.